Two field experiments were conducted on a mixed species(Lolium perenne L., Poa annua L., Trifolium repens L.) sward, one in the summer and the other in the autumn of 1979. The objective of the study was to examine the relationship between sward conditions and herbage production. This was done by estimating tissue fluxes in swards maintained as near as possible in a steady state with reference to herbage mass under continuous but variable stocking management with sheep. In the first experiment four separate swards were maintained at herbage mass levels of 500, 700, 1000 and 1700 kg 0M ha ^ for the period May to July inclusive. Tissue flows were estimated from measurements on individually identified grass tillers and clover stolons in the field and determinations of tiller and stolon population densities. Rates of herbage growth and senescence compensated each other in such a way that their resultant, net production, was relatively constant over the range of herbage mass from 800 to 1800 kg OM ha ^. Variations within and between treatments in the utilisation of the three plant species were attributable to the distribution of their foliage within the sward canopy. The herbage intake and intake per bite of the ewes increased with the level of herbage mass maintained, but the time the ewes spent grazing increased,reached a maximum in the 1000 kg OM ha-1 sward, and then declined as the level of maintained herbage mass was increased. In the second experiment, deliberate changes in herbage mass were induced in an attempt to increase the rate of net production above that obtainable in swards maintained at a constant herbage mass. Swards were maintained at either high (1700 kg OM ha HH) or low(700 kg OM ha?\ LL) herbage mass, or manipulated from one state to the other over a period of three weeks in August - September (HL and LH).Rates of growth and senescence compensated each other in such a way that net production was similar in the HH, LL, and LH swards, but inthe HL sward rates of herbage growth, senescence and net production were all reduced. A conceptual model of the sward-animal interface was developed,based upon the results of experiment 1, in which levels of green herbage mass control the rates of herbage growth and senescence,the level of herbage intake and the botanical composition of the diet. A simulation exercise based on this conceptual model was used to examine the implications of basing management decisions on herbage mass or a derivative such as lamina mass. This exercise indicated that lamina was likely to be more useful than herbage mass as the basis for decision making and highlighted the potential of tissue flow analysis as a means of evaluating alternative management systems.